Dental x-ray tube stabilizer having a control switch in the filament circuit



p 8, 1970 L. L. WEISGLASS ET AL ,5

DENTAL X-RAY TUBE STABILIZER HAVING A CONTROL SWITCH IN THE FILAMENT CIRCUIT Filed Jan. 25, 1968 IAM c FIRING OF CONTROL SILICON RECTIFIER 24a ATTORNE United States Patent 3,527,947 DENTAL X-RAY TUBE STABILIZER HAVHIG A CONTROL SWITCH IN THE FILAMENT CIRCUIT Louis L. Weisglass, New York, and William C. Pmnell,

Elmont, N.Y., assignors to S. S. White Company, Philadelphia, Pa., a corporation of Pennsylvania Filed Jan. 25, 1968, Ser. No. 700,442

Int. Cl. Hg 1/32; G03b 41/16 US. Cl. 250-103 8 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION Several systems are known in the art for maintaining the dischange current across an X-ray tube constant regardless of fluctuations in the line voltage supply to the high tension transformer and particularly to the low tension transformer for supplying heating current to the X-ray tube filament. Systems of this general type are shown for example in US Pats. No. 2,319,378 and No. 2,494,218 both previously granted to one of the present applicants.

These prior systems proved to be quite satisfactory using magnetic amplifiers with their D.C. winding receiving amplified impulses of the discharge current across the X-ray tube which thereby changed the impedance of its D.C. coils and consequently regulated the filament voltage to thus keep the discharge current across the X-ray tube constant. The difiiculty with these prior type stabilizers resided in the fact that they were quite bulky and rather expensive to produce. Such fact was not particularly a detriment because these prior stabilizer systems Were utilized with rather large and expensive therapeutic or radiographic X-ray machines. Due to size and expense they were accordingly inapplicable for the stabilization of small compact units such as dental X-ray tube apparatus.

SUMMARY OF THE PRESENT INVENTION It is accordingly the object of the present invention to produce a relatively small and inexpensive stabilizer system particularly for small dental X-ray machines.

Another object of the present invention is the provision of a stabilizer system which is readily adaptable to relatively small controls for dental X-ray apparatus and relatively inexpensive to produce.

Another object of the present invention is the provision of a stabilizer for dental X-ray tube apparatus that automatically keeps the discharge current of the X-ray tube constant without utilizing voltage stabilization and compensating circuits.

A further object of the present invention is to provide a stabilizer system which is rapidly responsive yet small and compact thus making it inexpensive to produce.

The foregoing objects together with others which will become apparent to those skilled in the art as the following description proceeds are achieved in accordance with the present invention by the provision of a stabilizer system employing a voltage sensitive device responsive 3,527,947 Patented Sept. 8, I970 ice to a predetermined increase in voltage of the high tension energizing transformer for the X-ray tube whereby such voltage sensitive device operates to cause regulation of the supply voltage for the X-ray tube filament during one half-wave of the alternating current cycle and such sensitive device being responsive to the discharge current across the X-ray tube during the other half-Wave of the alternating current cycle.

The present invention may be more fully appreciated by reference to the accompanying drawing wherein:

FIG. 1 is a schematic diagram of the circuitry forming one embodiment which the present invention may take,

FIG. 2 is a diagrammatic illustration of one complete wave of the primary voltage across the X-ray tube filament transformer and showing the proper moment of conduction of the solid state device in the stabilizer circuit for maintaining correct current through the X-ray tube,

FIG. 2a that is a diagrammatic illustration similar to FIG. 2 but showing the moment in the X-ray tube voltage wave that the solid state device fires when the X-ray tube current tends to rise too high,

FIG. 2b is a diagrammatic illustration similar to FIGS. 2 and 2b but showing the moment of firing of the solid state device in the stabilizer circuit when the X-ray tube current tends to fall too low,

FIG. 3 is a schematic diagram similar to that of FIG. 1 but showing a slight modification which the stabilizer circuitry of the present invention may take,

FIG. 4 is a schematic diagram similar to that of FIG. 3 except that in this concept of the present invention, the regulation of the discharge current of the X-ray tube and hence stabilization, is extended to both half-waves of the alternating current cycle by the stabilizer responding to X-ray tube discharge current during one half-wave as in FIG. 3 and responding to fluctuating line voltage during the remaining half-Wave,

FIG. 5 is a diagrammatic illustration similar to FIG. 2 but showing complete stabilization over the full cycle, one half-wave controlled by the discharge current of the X-ray tube and the remaining half-wave controlled by the line voltage,

FIG. 5a is a diagrammatic illustration similar to FIG. 5 showing stabilization when the supply line voltage rises too high, and

FIG. 5b is a diagrammatic illustration similar to FIG. 5a but showing stabilization when the supply line voltage falls too low.

Referring now to the drawings in detail a small dental X-ray tube 5 is shown which receives energy from a high voltage transformer 6 and has its cathode heated by current from a low voltage heating transformer 7. The dental X-ray tube 5 is of the customary type which has a kilovolt range from a minimum of about 50 kv. to a maximum of about kv. depending upon the magnitude of the supply voltage setting of the high tension trans former 6. The filament voltage for the X-ray tube cathode is supplied by what is termed a constant voltage transformer 7 so that such voltage supposedly maintains the discharge current of the X-ray tube constant throughout its kilovolt range of 50 kv. to 90 kv. but from a practical standpoint this is not the case.

Since the wave shape of the discharge current charges with greater saturation of the filament transformer at 90 kv. this inherently increases the average milliamperage output at high kv. Moreover, in a dental X-ray tube unit both the high tension transformer and the filament transformer are built into a small tube head and hence in close proximity to each other. Since the magnetic saturation of the high tension transformer increases with higher voltage, a percentage of its magnetic flux leaves the iron core and migrates through the insulating oil striking 3 through the iron core of the closely adjacent filament transformer, thereby changing the voltage output of the latter as a function of the kilovolt setting. This effect is counteracted by the special compensating circuits of the present invention which operate to maintain the discharge current of the X-ray tube constant at all kilovolt settings.

As shown in FIG. 1, the high tension transformer 6 is provided with a primary winding 8 connected to the usual domestic source of supply L1-L2 and a pair of high voltage secondary windings 9 and 10, with the outer end of winding 9 being connected to the anode of the X-ray tube 5, while the outer end of the secondary winding 10 is connected to the cathode of such X-ray tube. The remaining end of the high voltage secondary winding 9 is connected to a constant voltage Zener diode 12 in the stabilizer circuit and the remaining end of high voltage secondary winding 10 is connected through ground G to a resistor 13 also forming a part of the stabilizer circuit. The filament heating transformer 7 has its lowvoltage secondary winding 14 connected to the cathode of the X-ray tube 5 as above-mentioned while its primary winding 15 is connected through a variable resistor 16 to the source of supply Ll-LZ.

The stabilizer circuit also includes a unijunction transistor 17 connected by conductors 18 and 19 to the constant voltage diode 12 through the primary winding of a pulse transformer 20. In addition such stabilizer circuit includes a variable resistor 22 in series with a capacitor 23 with adjacent ends of each being connected to the base of unijunction transistor 17. Also as shown in FIG. 1, the secondary winding of pulse transformer is connected to a silicon control rectifier (S.C.R.) 24 which latter in turn is connected across the primary voltage supply for the cathode heating transformer 7.

During operation of the self-rectifying X-ray tube 5 the discharge current flows therethrough sixty times per second during the forward half-cycle of the A.C. current wave. Since the resistor 13 in the stabilizer circuit is thus subject to this current flow, a voltage drop occurs across such resistor 13 accompanied by a constant voltage drop across the series connected constant voltage diode 12. The constant voltage thus across this diode 12 is supplied to unijunction transistor 17 through the primary of pulse transformer 20. At the same time current flows through resistor 13, the parallel connected capacitor 23 is charged at a rate controlled by the setting of variable resistor 22. The latter is so adjusted as to cause the unijunction transistor 17 to fire in the middle of the discharge cycle of the X-ray tube.

This is shown more specifically in the graphic illustration of FIG. 2 wherein the forward portion of the discharge cycle for a period of 4.15 milliseconds is shown by the shaded portion of the curve above the base line from 0 to 90. At this point the S.C.R. rectifier 24 fires for the remainder of the 8.3 milliseconds wave duration when it returns to 180, with the remaining or reverse half-wave of the cycle, shown by the shaded portion below the base line, being entirely suppressed by the selfrectifying X-ray tube 5 itself.

The discharge current thus flowing through the stabilizer circuit has two functions by first causing discharge of the capacitor 23 and thus conditioning it for the following charging cycle and, secondly, such discharge current produces a pulse in the secondary winding of pulse transformer 20 of sufiicient magnitude to fire the silicon control rectifier 24. This in turn short-circuits the primary supply voltage to the filament heating transformer 7 for approximately one-quarter of a cycle thus reducing the filament voltage Vpf of the X-ray tube.

The foregoing describes the operation of the stabilizer circuit under conditions of correct X-ray tube current. In the event, however, that such current tends to rise too high then the supply voltage for variable resistor 22 and series connected capacitor 23 likewise rises to the same magnitude, resulting in the capacitor 23 being charged at a faster rate and consequently unijunction transistor 17 and S.C.R. rectifier 24 fire earlier. This is shown by the graphic illustration of FIG. 2a, while the converse condition when the X-ray tube current is too low, is shown in FIG. 2b wherein the transistor 17 and rectifier 24 fire later after the AC. half-wave has passed its peak of FIG. 3 shows a slight modification of the present invention which differs from that above described merely in the substitution of a trigger diode 25 for the unijunction transistor 17 of FIG. 1 and which substitution also renders the constant voltage Zener diode 12 of FIG. 1 superfluous. In the operation of the stablizer circuit, as shown in the modification of FIG. 3, only one side of the trigger diode 25 actually fires since it is subjected of course to only onebalf wave of the alternating current cycle, although such type trigger diode can be utilized for passage of current first in one direction and then in the other whenever desired in an appropriate circuit. Also, the trigger diode 25 may be replaced by a neon glow lamp, if desired, since the most precise voltage responsive device whatever its design when utilized in the circuit as herein shown and described will produce the best stabilization of the X- ray tube discharge current.

From the foregoing description of the present invention it should be obvious to those skilled in the art, that the stabilization effected by the circuits of FIGS. 1 and 3 is controlled by the discharge current of the X-ray tube, which fiows therethrough only during alternate or positive half-waves of the alternating current cycle. Consequently in order to achieve still greater regulation over the entire alternating current cycle a stabilized circuit as shown in FIG. 4 may be utilized. By reference now to this latter figure it will be noted that so far as stablization during the flow of discharge current through the X-ray tube 5 is concerned, the circuit of FIG. 4 is identical to that as shown and above described relative to FIG. 3 up to the dotted line AA of FIG. 4.

Accordingly, to effect stabilization during the remaining or off half-cycle, when no current flows through the X-ray tube, a portion of the stabilizer circuit of FIG. 3 is duplicated in the circuit of FIG. 4 by employment of some of the same components. Fore example, a controlled silicon rectifier 24a parallels that of S.C.R. 24 but which is reversely connected to one side of the secondary winding of pulse transformer 20 and hence to one side of the source of voltage supply L1 for primary winding 15 of the low-voltage heating transformer 7. Such addition to the stabilizer circuit of FIG. 4 also includes a trigger diode 25a together with a series connected var-iable resistance 22a and capacitor 23a which series circuit is connected across S.C.R. 24a while a protective diode 26 is shunted across S.C.R. 24a for protecting it from high negative voltage at its trigger electrode.

By virtue of such additional components, as shown and described in FIG. 4, when the sine wave of the supply voltage is positive (and which at this particular instant results in a negative reverse voltage at S.C.R. 24) the capacitor 23a will be charged through variable resistor 22a to a positive potential and finally discharge through trigger diode 25a to S.C.R. 24a causing the latter to fire sooner or later depending upon whether the supply voltage for the winding 15 of low voltage transformer 7 is higher or lower. This results in complete stabilization during the entire alternating current cycle as can be more fully appreciated from the graphic illustrations of FIGS. 5, 5a and 5b.

For example, FIG. 5 shows the wave form of the forward half-wave of the alternating current cycle resulting from control of the milliamperage of current through the X-ray tube 5 in the same manner as depicted by FIG. 2. Also, however, FIG. 5 depicts the wave form during the reverse half-wave when no current flows through the self-rectifying X-ray tube 5 and wherein stabilization is effected by control of the line voltage supplied by L1- L2 to the primary winding 15 of low voltage transformer 7 so that such line voltage is effectively maintained uniform at approximately 117 volts. Should such supply voltage tend to rise to say 130 volts the forward half-wave taks the form shown in FIG. 5a above the base line, while during the reverse half-wave it takes the form shown in such figure below the base line. In the same manner FIG. 5]) depicts the forward and reverse half-waves of the full alternating current cycle in the event the supply voltage for transformer 7 falls to say 105 volts.

It should accordingly become obvious to those skilled in the art that by the phase shifting operation of the stabilizer system of th present invention stabilization is effected during the forward half-wave of the alternating current cycle and, if desired, such stabilization is further effected during the remaining half-wave. This is accomplished by controlling th discharge current through the X-ray tube during one half-wave and by controlling the supply voltage to the filament heating transformer during the remaining half-wave of such cycle so that such complete regulation takes place sixty times per second. Also, since the unijunction transistor as shown and described herein functions as a voltage detecting device, it should be obvious that other voltage detectors, such as a thyratron tube, or a glow lamp could be substituted for such unijunction transistor.

The effectiveness of the stabilizer of the present invention may be further appreciated by pointing out that in the circuit as shown without stabilization a supply voltage of 117 volts resulted in X-ray tube discharge current of 10 milliamperes. However, when the supply voltage rose to 130 volts the discharge current increased nearly three times to 28 milliamperes and a fall in line voltage to 105 volts produced a discharge current of 2.8 milliamperes. In contrast to such wide variation, and using the same power supply at 117 volts and X-ray tube current of 10 ma. but utilizing the stabilizer circuit of the present invention, a rise in supply voltage to 130 volts produced an X-ray tube current of only 10.8 ma. and conversely a decrease in supply voltage to 105 volts resulted in an X-ray tube current of 9.2 ma. From these figures it should be apparent that the stabilizer circuit of the present invention is exceedingly efiicient and dependable in maintaining the discharge current through the X-ray tube substantially constant during the entire alternating current cycle.

Although several embodiments of the present invention have been herein shown and described, it is to be understood that still further modification thereof may be made Without departing from the spirit and scope of the appended claims.

We claim:

1. In combination with a self rectifying high voltage X-ray tube having a thermionic cathode heated by a filament, a low voltage transformer whose secondary is coupled to the filament, and a high voltage transformer whose secondary is coupled across the anode and cathode of said X-ray tube, a current stabilizer for maintaining the discharge current through said X-ray tube substantially constant comprising:

means for monitoring the half wave pulses of output current of said X-ray tube including an R-C network which charges with each positive half cycle of output current at a rate proportional to the amplitude thereof and then discharges,

means constituting an electrical switch interposed across the primary of said low voltage filament transformer, and

means triggered by the discharge of said R-C network for actuating the closing of said electrical switch and effecting conduction thereof for the remainder of respective positive half cycles of A.C. input voltages to said filament transformer corresponding to the half wave pulses of output current,

said R-C network being so constituted to discharge at a time corresponding substantially to the midpoint of each positive half cycle of input voltage when the output current of the X-ray tube is at its prescribed normal level,

whereby an output current level in excess of the prescribed normal level causes of the electrical switch to be triggered at a time before the midpoint of said positive half cycle while an output current level below the prescribed normal level causes the electrical switch to be triggered at a time after the midpoint of said positive half cycle.

2. The current stabilizer of claim 1 in conjunction with means for monitoring the AC. input voltage to said low voltage filament transformer including a second R-C network which charges with each negative half cycle of input 'voltage at a rate proportional to the amplitude thereof and then discharges,

means constituting a second electrical switch interposed across the primary of said low voltage filament transformer, and

means triggered by the discharge of said second R-C network for actuating the closing of said second electrical switch and effecting conduction thereof for the remainder of the corresponding negative half cycle of input voltage,

said second IR-C network being so constituted to discharge at a time corresponding substantially to the midpoint of each negative half cycle of input voltage when the input voltage is at a predetermined rated level,

whereby an input voltage in excess of the predetermined level causes said second electrical switch to be triggered at a time before the midpoint of said negative half cycle while an input voltage below the predetermined level causes said second electrical switch to be triggered at a time after the midpoint of said negative half cycle thereby effecting stabilization when the X-ray tube is both conducting and non-conducting.

3. The stabilizer of claim .1 wherein each of said R-C networks includes a timing capacitor and a variable resistor in series therewith.

4. The stabilizer of claim 3 wherein each of said electrical switches comprises a gate controlled electronic conductive element.

5. The stabilizer of claim 4 wherein each of said means for actuating the respective gate controlled conductive elements comprises a pulse triggered electrical firing device having an output coupled to the gate electrodes of the respective gate controlled conductive elements.

6. The stabilizer of claim 5 wherein each of said gate controlled conductive elements comprises a silicon controlled rectifier in opposed relationship to each other.

7. The stabilizer of claim 6 wherein each of said pulse triggered electrical firing devices comprises a trigger diode.

8. The stabilizer of claim 3 wherein said means for monitoring the output current of said X-ray tube comprises a resistor across which the output current produces a voltage drop in passing therethrough, and the variable resistor and timing capacitor of said first R-C network being connected across said resistor.

References Cited UNITED STATES PATENTS 3,122,677 2/1964 Flieder 315-340 X 3,275,883 9/1966 Watters 315-106 3,406,286 10/1968 Bross 250 ARCHIE R. BORCHELT, Primary Examiner A. L. BIRCH, Assistant Examiner US. Cl. X.R. 

